The present invention relates to a method for changing an operating state of an electric machine and a device for changing the operating state of an electric machine to a free-wheeling mode.
Electric machines, such as, for example, permanently excited synchronous machines, are used in numerous technical areas. Such permanently excited synchronous machines are, for example, used in motor vehicles. For safety reasons, provisions must be taken for a safe operating state in the event of a fault. An option for a safe operating state is, for example, the so-called active short-circuit. In this case, the connections of the electric machine are short-circuited by means of suitable switching elements. A further safe operating state is the so-called freewheel. In the freewheeling mode, all of the connections of the electric machine are disconnected from one another, and an active actuation of the electric machine with voltage does not take place.
The German patent application DE 10 2011 081 173 A1 discloses a device and a method for setting operating states of an inverter which actuates an electric machine. In this case, the electric machine can be switched by means of the inverter to a freewheeling mode or to an active short circuit depending on the rotational speed of said electric machine.
During the active short circuit, relatively high currents can flow through the switching elements which connect the individual phase connections of the electric machine to one another. If the electric machine is now to be changed from the active short circuit to the freewheeling mode, the switching elements between the individual phase connections of the electric machine are then preferably opened if no or only a low amount of current flows through the respective switching elements. To this end, the currents have to be monitored which flow through the respective switching elements.
There is therefore a need for a method and a device for a simple and cost effective change of the operating state of an electric machine to the freewheeling mode.